1. Technical Field
The present invention relates to an apparatus and method for mechanically reducing the internal pressure of a column in a refinery or petrochemical process. More particularly, the present invention relates to an apparatus and method for mechanically reducing the internal pressure of a column in a refinery or petrochemical process, in which, when the internal pressure of the column increases or exceeds a predetermined level, the supply of a heat source is blocked, whereby the internal pressure of the column is maintained at the predetermined level or lower, thus reducing the capacity of a flare stack.
2. Description of the Related Art
Generally, in refinery and petrochemical plants, a flare stack must be essentially mounted. The flare stack is a device for automatically releasing gas from a process column to the atmosphere and burning it so as to prevent explosion of the column when the internal pressure of the process column is abnormally increased due to interruption of the supply of power or cooling water or because of the outbreak of a fire.
In a conventional system, gas is discharged from the column to the atmosphere using a safety valve. Specifically, when the internal pressure of the column exceeds a pressure set for the safety valve, the safety valve opens so that the gas is discharged to the atmosphere, thus reducing the internal pressure of the column, thereby preventing the explosion of the column.
The gas released from the column through the operation of the safety valve is collected into a header line, and passes through a seal drum and a knockout drum for separating and removing a liquid mist, thereby discharging the burned gas to the atmosphere.
However, in the case where the quantity of gas ejected further increases owing to the construction of more production facilities or the replacement of the facilities in order to increase throughput of conventional equipment, it may exceed the design capacity of the flare stack which uses only the above safety valve without using an additional device.
To solve this problem, another flare stack should be additionally provided. Also, in the case where plants are newly constructed, attempts to set the ejection itself to a low level thus reducing the investment cost of the flare stack are made.
Therefore, proposed as an alternative therefor is a safety instrumented system (SIS). A SIS is typical and is commercially available in domestic and foreign countries. This SIS controls the process state using an electrical system such as a computer. U.S. Pat. No. 7,289,861 discloses a system for controlling a process plant using the SIS.
The SIS has been used to reduce the internal pressure of the process column in such a manner that the operating pressure is set to a level equal to or lower than the set pressure of the safety valve, and thus the internal pressure of the column is reduced in advance before operation of the safety valve, so that the amount of gas discharged to the flare stack is decreased, thereby achieving the reduction of the handling capacity of the flare stack.
The SIS includes a pressure sensing system, a TMR (triple modular redundant) controller and an emergency shutdown valve system. FIG. 1 schematically shows the process using the SIS.
With reference to FIG. 1, the SIS is applied to the process in which a light gas of a material heated in the column is discharged through an upper portion of the column, liquefied in a condenser and then transferred to a drum, whereas a heavy component thereof is discharged through a lower portion of the column. In a normal operating state, because the pressure is maintained at a risk level or less, there is no need to operate the safety valve or the SIS.
However, in the case where there occurs an abnormal situation, for example, interruption of the supply of power or cooling water, heat may accumulate in the apparatus attributable to the continuous supply of a heat source such as steam into the column and stopping of the operation of the condenser, undesirably increasing the internal pressure of the column.
In this case, the SIS transmits analog signals to the TMR controller from the pressure sensing system located at the upper portion of the column. When the internal pressure of the column reaches the preset pressure, the TMR controller transmits the signals to the emergency shutdown valve, thus closing the shutdown valve.
Therefore, the pressure of the apparatus is preset to the level equal to or lower than the operating pressure of the safety valve, thus reducing unnecessary flaring. As a result of reducing such unnecessary flaring, an additional need to increase the capacity of the flare stack is obviated, thus achieving cost saving effects and simplifying the plant design.
Although the SIS requires a lower investment than conventional methods, it requires equipment investment due to the purchase of devices. Moreover, in order to certify suitability of this system by an approval authority, reliability equivalent to that of the safety valve should be imparted. However, the SIS does not have high reliability due to the use of electrical signals. Also, because there is a probability of failure of one among ten devices using the SIS, the greatest amount among those amounts of ejected gas is regarded as being ejected. Taking into consideration such an ejected amount, the capacity of the flare stack should be set. So, the reduction effect using the SIS is decreased. This is because the shutdown valve is controlled using electrical signals, undesirably causing problems in which reliability of the SIS does not reach that of the safety valve using mechanical signals.